Shock absorbers are used in conjunction with automotive suspension systems to absorb unwanted vibrations which occur during driving. To absorb these unwanted vibrations, shock absorbers are generally connected between the sprung portion (body) and the unsprung portion (suspension) of the automobile. A piston is located within a pressure tube of the shock absorber and the pressure tube is normally attached to the unsprung portion of the vehicle. The piston is normally attached to the sprung portion of the vehicle through a piston rod which extends through the pressure tube. The piston divides the pressure tube into an upper working chamber and a lower working chamber. The shock absorber, by restricting fluid flow between the upper and lower working chambers, produces a damping force that counteracts the vibration that would otherwise be transmitted from the unsprung portion of the vehicle to the sprung portion of the vehicle.
Spring devices are implemented with the shock absorbers to resiliently support the vehicle on the suspension system. Exemplary spring devices include coil springs, torsion bars and air springs. As the vehicle load increases the spring devices compress. The dampening capability of the shock absorbers, however, remains constant regardless of the vehicle load. While a constant dampening ability may be acceptable in some applications, other applications would benefit from a shock absorber whose dampening characteristics vary with vehicle load.